Laser thermal annealing of lightly doped silicon substrates

ABSTRACT

Apparatus and method for performing laser thermal annealing (LTA) of a substrate using an annealing radiation beam that is not substantially absorbed in the substrate at room temperature. The method takes advantage of the fact that the absorption of long wavelength radiation (1 micron or greater) in some substrates, such as undoped silicon substrates, is a strong function of temperature. The method includes heating the substrate to a critical temperature where the absorption of long-wavelength annealing radiation is substantial, and then irradiating the substrate with the annealing radiation to generate a temperature capable of annealing the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.10/287,864, filed on Nov. 6, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to laser thermal annealing, and inparticular relates to apparatus and methods for performing laser thermalannealing of substrates that do not efficiently absorb the annealingradiation beam at ambient temperatures.

2. Description of the Prior Art

Laser thermal annealing or LTA (also referred to as “laser thermalprocessing”) is a technique used to quickly raise and lower thetemperature of the surface of a substrate to produce a change inproperties. An example might include annealing and/or activating dopantsin the source, drain or gate regions of transistors used to formintegrated devices or circuits. LTA can also be used to form silicideregions in integrated devices or circuits, to lower poly-silicon runnerresistances, or to trigger a chemical reaction to either form or removesubstances from a substrate (or wafer).

LTA offers the possibility of speeding up the annealing cycle by afactor of 1000 over conventional annealing techniques, thereby virtuallyeliminating diffusion of dopant impurities during the annealing oractivation cycle used on silicon wafers. The result is a more abruptdopant profile and, in some cases, a higher level of activation. Thistranslates into higher-performance (e.g., faster) integrated circuits.

U.S. patent application Ser. No. 10/287,864 discloses performing LTA ofdoped silicon substrates using CO₂ laser radiation. The laser radiationis focused into a narrow line, which is scanned at constant velocity ina raster pattern across the substrate. However, this approach works wellonly on relatively heavily doped substrates (i.e., a dopantconcentration of about 3×10¹⁷ atoms/cm³ or greater), where theabsorption length of the laser radiation in the doped silicon is lessthan or roughly comparable to the thermal diffusion length. Conversely,for lightly doped substrates (i.e., a dopant concentration of about1×10¹⁶ atoms/cm³ or less), the CO₂ laser radiation passes through thesubstrate without imparting appreciable energy to the substrate.

What is needed therefore is way to efficiently perform LTA of lightlydoped silicon substrates using radiation that otherwise passes throughthe substrate without heating, such as CO₂ laser radiation having awavelength of 10.6 μm.

SUMMARY OF THE INVENTION

A first aspect of the invention is an apparatus for performing laserthermal annealing of a substrate having a surface. The apparatusincludes a laser capable of generating continuous annealing radiationhaving a wavelength that is not substantially absorbed by the substrateat room temperature. The apparatus also includes an annealing opticalsystem adapted to receive the annealing radiation and form an annealingradiation beam that forms a first image at the substrate surface, andwherein the first image is scanned across the substrate surface. Theapparatus further includes a heating device for heating at least aportion of the substrate to a critical temperature such that theannealing radiation beam incident upon the heated portion issubstantially absorbed near the surface of the substrate duringscanning. In an example embodiment heating a portion of the substratecan be done using a short-wavelength laser diode beam that immediatelyprecedes the long-wavelength annealing beam.

A second aspect of the invention is a method of laser thermal annealinga substrate. The method includes providing an annealing radiation beamfrom a laser having a wavelength that at room temperature is notsubstantially absorbed by the substrate, and heating at least a portionof the substrate to a critical temperature such that the annealingradiation beam is capable of being substantially absorbed near thesurface of the substrate at said heated portion. The method alsoincludes initiating a self-sustaining annealing condition by heating aportion of the substrate surface immediately in advance of scanning theannealing radiation beam over the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of an example embodiment of the LTAapparatus of the present invention that includes an LTA optical systemalong with a silicon substrate being processed by the system, whereinthe LTA apparatus includes a heated chuck to support and pre-heat thesubstrate, and an optional heat shield surrounding the chuck to reduceradiation coupling to the rest of the apparatus and to promote efficientsubstrate heating;

FIG. 1B is a cross-sectional view of an embodiment of the LTA apparatusof the present invention similar to that shown in FIG. 1A, that includesa heated enclosure surrounding the substrate for pre-heating thesubstrate;

FIG. 1C is a cross-sectional view of an embodiment of the LTA apparatusof the present invention similar to that shown in FIG. 1A, wherein theheated chuck and optional heat shield are replaced by an optical heatingsystem adapted to preheat at least a portion of the substrate using apreheating radiation beam;

FIG. 2 is a plot of the absorption path length L_(A) (μm) in an undopedsilicon substrate versus the substrate temperature T_(S) (° C.) for a10.6 μm wavelength annealing radiation beam, along with a plot of thediffusion length L_(D) associated with the radiation beam having a 200μs dwell time, versus substrate temperature T_(S) (° C.);

FIG. 3 is a computer simulation of the substrate temperature profile asa function of depth (μm) and annealing radiation beam position (μìm)illustrating the “hot spot” formed in the substrate by the annealingradiation beam associated with the self-sustaining annealing condition;

FIG. 4A is a schematic diagram showing an example embodiment of therelative intensities and beam profiles of the pre-heating and annealingradiation beams as a function of position on the substrate surface;

FIG. 4B is a close-up cross-sectional view of the substrate illustratinghow heat from the preheating radiation beam imaged in front of theannealing radiation beam promotes absorption of the annealing radiationbeam in the substrate to effectuate the self-sustaining annealingcondition;

FIG. 5 is a plot of the maximum substrate temperature T_(MAX) (° C.)created by irradiating a heavily doped silicon substrate with theannealing radiation beam having a wavelength of 10.6 μm, versus theincident power P_(I) (W/cm) of the annealing radiation beam;

FIG. 6 is a plot, obtained by finite-element simulation, of the maximumsubstrate temperature T_(MAX) (° C.) as a function of the initialsubstrate temperature T_(I) for different incident powers P_(I) of theannealing radiation beam for an undoped substrate;

FIG. 7 is a plot of the absorption length L_(A) (μm) of the 780 nmpreheating radiation beam in silicon as a function of substratetemperature T_(S) (° C.);

FIG. 8A is a cross-sectional view of an embodiment of the optical relaysystem of FIG. 1C, as viewed in the Y-Z plane;

FIG. 8B is a cross-sectional view of the embodiment of the optical relaysystem of FIG. 1C and FIG. 8A, as viewed in the X-Z plane;

FIG. 9A is a close-up cross-sectional view in the X-Z plane of theheating radiation source and the cylindrical lens array;

FIG. 9B is a close-up cross-sectional view in the Y-Z plane of theheating radiation source and the cylindrical lens array;

FIG. 10A is a close-up schematic diagram of the preheating radiationsource, relay lens and preheating radiation beam at normal incidence tothe substrate and further including a polarizer and quarter waveplatearranged in the preheating radiation beam to reduce the amount ofpreheating radiation reflected from the substrate and returning to thepreheating radiation source; and

FIG. 10B is a close-up schematic diagram of the preheating radiationsource, relay lens and preheating radiation beam at near-normalincidence to the substrate and further including a polarizer and Faradayrotator arranged in the preheating radiation beam to reduce the amountof preheating radiation scattered from the substrate and returning tothe preheating radiation source.

The various elements depicted in the drawings are merelyrepresentational and are not necessarily drawn to scale. Certainproportions thereof may be exaggerated, while others may be minimized.The drawings are intended to illustrate various implementations of theinvention, which can be understood and appropriately carried out bythose of ordinary skill in the art.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to laser thermal annealing (LTA) ofsubstrates and in particular relates to apparatus and methods forperforming LTA of lightly doped silicon wafers (substrates). The term“lightly doped” means herein a dopant concentration of about 10¹⁶atoms/cm³ or less. The dopant concentration in the substrate may be thatassociated with normal substrate production to achieve a desiredresistivity level and substrate type (i.e., N-type or P-type).

In the description below, a generalized embodiment of the LTA apparatusof the present invention is described, along with a description of the“self-sustaining annealing condition” sought to be created by thepresent invention. This is followed by various example embodiments ofthe invention. The invention is further explained in connection with anumber of different substrate temperature plots that illustrate keyproperties of the absorption of radiation by silicon substrates. Methodsof determining the appropriate power level in the preheating radiationbeam are then discussed, followed by an example of a heating lens usedin an example embodiment to heat the substrate with a preheatingradiation beam. The preferred scanning and orientation of the preheatingand annealing radiation beams are then described in detail.

I. Generalized LTA Apparatus

FIG. 1A is a cross-sectional view of an embodiment of an LTA apparatus 8of the present invention, along with a substrate 10 to be annealed.Substrate 10 has an upper surface 12 and a body (bulk) region 16 that is“undoped,” or strictly speaking, more lightly doped than the very smalljunction regions or devices that typically contain very high dopinglevels only in an extremely shallow region. The reference letter Ndenotes the normal to substrate upper surface 12. In an exampleembodiment, substrate 10 is a silicon wafer.

LTA apparatus 8 includes an LTA optical system 25 having an annealingradiation source 26 and an LTA lens 27 arranged along an optical axisA1. Lens 27 receives continuous (i.e., non-pulsed) annealing radiation18 from annealing radiation source 26 and creates a continuous annealingradiation beam 20 that forms an image 30 (e.g., a line image) atsubstrate surface 12. Annealing radiation beam 20 is incident uppersurface 12 at an incident angle θ₂₀ as measured relative to surfacenormal N and optical axis A1.

Arrow 22 indicates an example direction of motion of annealing radiationbeam 20 relative to substrate surface 12. Substrate 10 is supported by achuck 28, which in turn is supported by a movable stage MS operativelyconnected to a stage driver 29 that causes the stage (and hence thesubstrate) to move at select speeds and directions relative to annealingradiation beam 20 or some other reference. The scanning movement ofmovable stage MS is indicated by arrow 22′. In an example embodiment,stage MS is capable of moving in at least two dimensions.

In an example embodiment, LTA apparatus 8 includes a reflected radiationmonitor M1 and a temperature monitor M2. Reflected radiation monitor M1is arranged to receive radiation reflected from substrate surface 12, asindicated by radiation 20R. Temperature monitor M2 is arranged tomeasure the temperature of substrate surface 12, and in an exampleembodiment is arranged along the surface normal N to view the substrateat normal incidence at or near where image 30 is formed by annealingradiation beam 20. Monitors M1 and M2 are coupled to a controller(discussed immediately below) to provide for feedback control based onmeasurements of the amount of reflected radiation 20R and/or themeasured temperature of substrate surface 12, as described in greaterdetail below.

In an example embodiment, LTA apparatus 8 further includes a controller32 operatively connected to annealing radiation source 26, stage driver29, monitors M1 and M2, as well as to an optional monitor M3 containedin lens 27 that serves as an incident power monitor. Controller 32 maybe, for example, a microprocessor coupled to a memory, or amicrocontroller, programmable logic array (PLA), field-programmablelogic array (FPLA), programmed array logic (PAL) or other control device(not shown). The controller 32 can operate in two modes: 1) open-loop,wherein it maintains a constant power delivered to substrate 10 byannealing radiation beam 20 along with a constant scan rate via stagedriver 29; and 2) closed-loop, wherein it maintains a constant maximumtemperature on substrate surface 12 or a constant power absorbed in thesubstrate. The maximum substrate temperature varies directly with theabsorbed power and inversely as the square root of the scan velocity.

In an example embodiment a closed loop control is used to maintain aconstant ratio of absorbed power in annealing radiation beam 20 incidentthe substrate, to the square root of the scan velocity. i.e., if P₂₀ isthe amount of power in annealing radiation beam 20 and P₃₀ is thereflected power then the absorbed power is P_(a)=P₂₀−P₃₀. If V is thescan velocity of substrate 10 relative to the annealing radiation beam,then the ratio P_(a)/V^(1/2) is kept constant to maintain a constanttemperature indirectly.

For closed loop operation based on a direct maximum temperaturemeasurement, controller 32 receives a signal (e.g., an electricalsignal), such as the maximum substrate temperature via signal S2 fromtemperature monitor M2 and controls either the incident power or thescan rate to maintain a constant maximum substrate temperature. Theabsorbed power P_(a) is obtained by subtracting power P₃₀ in reflectedannealing radiation beam 20R via signal S1 generated by reflectedradiation monitor M1 from the incident power P_(I) of annealingradiation beam 20 obtained from sampling a portion of the annealingradiation beam via signal S4.

Further, controller 32 is adapted to calculate parameters based on thereceived signals and input parameters (e.g., desired absorbed powerlevel and dwell time). The controller 32 is also coupled to receive anexternal signal S3 from an operator or from a master controller (notshown) that is part of a larger assembly or processing tool. Thisparameter is indicative of the predetermined dose (amount) of annealingradiation 20 to be supplied to process the substrate or the maximumsubstrate temperature desired. The parameter signal(s) can also beindicative of the intensity, scan velocity, scan speed, and/or number ofscans to be used to deliver a predetermined dose of annealing radiation20 to substrate 10.

In an example embodiment, annealing radiation source 26 is a CO₂ laserso that annealing radiation beam 20 has a wavelength of 10.6 μm. Ingeneral, however, annealing radiation source 26 is any continuousradiation source that emits radiation having a wavelength notsubstantially absorbed by a substrate at room temperature, but issubstantially absorbed by the same substrate when the substrate, or asufficient portion of the top of the substrate, is at a highertemperature. In a preferred embodiment, annealing radiation source 26 isa laser.

LTA apparatus 8 is adapted to take advantage of the absorption ofannealing radiation beam 20 near the top of the substrate to efficientlyraise the temperature of the top of the substrate while leaving thetemperature of the body of the substrate substantially unchanged. Inother words, where the substrate is a semiconductor wafer, the presentinvention is directed to increasing the temperature of the wafer at ornear the surface where devices (e.g., transistors) are formed, ratherthan to heating the wafer body.

At ambient temperatures, however, lightly doped and undoped substratesare difficult to anneal because long-wavelength radiation beams passright through the substrate without heating the top surface appreciably.On the other hand, highly doped substrates are not difficult to annealbecause the incident annealing radiation is absorbed in the first 100microns or so of material and raises the temperature to the desiredannealing temperature.

The body (bulk) 16 of substrate 10, which does not absorb appreciableradiation from the beam and is not heated, serves to quickly cool thetop surface regions when the annealing radiation beam 20 is no longerapplied to the substrate. The present invention takes advantage of thefact that absorption of radiation in lightly doped silicon at certaininfrared wavelengths, such as the CO₂ laser wavelength of 10.6 μm,strongly depends on the substrate temperature. Once appreciableabsorption of annealing radiation beam 20 occurs, the substrate surfacetemperature increases, which results in stronger absorption, which inturn results in stronger heating of the substrate surface, and so on.

II. The Self-sustaining Annealing Condition

FIG. 2 is a plot of the absorption length L_(A) (μm) (vertical axis) ina silicon substrate versus the substrate temperature T_(S) (° C.) for10.6 μm wavelength radiation. Also included in the plot are points forthe diffusion length L_(D) (μm) for a 200 μs dwell time also as afunction of substrate temperature T_(S). The absorption length L_(A) isthe thickness it takes to attenuate the intensity of the annealingradiation beam 20 by 1/e. The thermal diffusion length L_(D) is thedepth to which an instantaneous surface temperature rise will propagateinto a material after a certain dwell time. Note that L_(A) and L_(D)have about the same value of ˜60 μm at a temperature T_(S)˜600° C.

The strong variation of absorption path length L_(A) with substratetemperature T_(S) creates two possible steady-state conditions, namely:(1) Annealing radiation beam 20 passes through the substrate withoutbeing substantially absorbed and thus does not produce substantialheating, or (2) annealing radiation beam 20 is substantially absorbednear substrate surface 12, thereby producing a “hot spot” at and justbelow the substrate surface corresponding to image 30 that travels withannealing radiation beam 20 as the beam moves (i.e., is scanned) overthe substrate surface.

FIG. 3 is a computer simulation of the substrate temperature (° C.)profile as a function of depth (μm) and annealing radiation beamposition (μm). The temperature profile is the hot spot (denoted by 31),which travels within the substrate and across substrate surface 12.Traveling hot spot 31 serves to preheat the region of substrate 10 infront of the advancing image 30 by thermal diffusion (see FIG. 4B,discussed below). The substrate preheating associated with thepropagation of hot spot 31 allows the radiation in annealing radiationbeam 20 to be efficiently absorbed near upper surface 12 as the beamsare scanned over the substrate surface. Steady-state condition (2) isthe one sought to be created using apparatus 8 and the accompanyingmethods of the present invention, and is referred to herein as the“self-sustaining annealing condition.”

The general method of creating a self-sustaining annealing conditionaccording to the present invention includes heating substrate 10 (orselect regions or portions thereof) to a critical temperature T_(C)(e.g., 350° C. or greater, as discussed in more detail below) so thatannealing radiation beam 20 is substantially absorbed by the substrate,i.e., is absorbed to the point wherein the self-sustaining annealingcondition is initiated.

The precise value of T_(C) depends on the temperature distributionwithin the substrate, its dopant concentration, and the annealingradiation beam intensity. Thus, in an example embodiment, the criticaltemperature T_(C) is determined empirically. This may include, forexample, measuring the maximum temperature produced by an annealingradiation beam for a test substrate having either a variety of initialtemperature conditions or a constant initial temperature condition and avariety of annealing and preheating radiation beam intensities. Thepreheating of substrate 10 to give rise to the self-sustaining annealingcondition can be accomplished in a number of ways. Several exampleembodiments of an LTA apparatus 8 that include heating devices forheating substrate 10 to practice the method of creating theself-sustaining annealing condition in lightly doped silicon substrates10 for the purpose of performing LTA are set forth below.

III. Heated Chuck Embodiment with Optional Heat Shield

With reference again to FIG. 1A, in an example embodiment chuck 28 isthermally conductive and includes a heating element 50 connected to apower source 52, which in turn is connected to, and controlled by,controller 32. A thermal insulating layer 53 surrounds the bottom andsides of the chuck 28 to limit unwanted heating of the stage and theloss of heat from the chuck.

In operation, controller 32 activates power supply 52, which in turnsupplies power to heating element 50. In response, heating element 50generates heat 56. In an example embodiment, the amount of generatedheat 56 is controlled by a temperature sensor 57 in the chuck andoperatively connected to power supply 52 (or alternatively to controller32) so that the chuck temperature is limited to a certain,predetermined, maximum value. Once the substrate is loaded onto thechuck its temperature quickly reaches the same temperature as the chuck.Typically, the chuck temperature T_(CH) is about 400° C.

In another example embodiment, apparatus 8 also optionally includes aheat shield 62 supported above substrate 12 so as to reflect heat 56back to the substrate. This results in more uniform heating of thesubstrate and less heating of the apparatus components on the oppositeside of the shield. In an example embodiment, heat shield 62 is agold-coated glass plate. Heat shield 62 includes an aperture 64 thatallows annealing radiation beam 20 to reach surface 12 of substrate 10.

IV. Heated Enclosure Embodiment

With reference to FIG. 1B, in another example embodiment, apparatus 8includes a heated enclosure 80 (e.g., an oven) having an interior region82 large enough to enclose both substrate 10 and chuck 28 or thesubstrate, chuck and stage MS. Enclosure 80 includes additional heatingelements 50 (preferably in addition to the one included in chuck 28)connected to power source 52. Power source 52 is connected to controller32. In an example embodiment, enclosure 80 includes a window or aperture84 that allows annealing radiation beam 20 to reach surface 12 ofsubstrate 10. Thermal insulating layer 53, discussed above in connectionwith FIG. 1A, is preferably present on the sides and bottom of the chuckto limit unwanted loss of heat from the chuck to the stage.

In operation, controller 32 activates power supply 52, which in turnsupplies power to heating elements 50. In response, heating elements 50generate heat 56, thereby raising the temperature of the chuck, thesubstrate and the immediate surroundings to a maximum criticaltemperature T_(C) of about 400° C. Enclosure 80 is preferably thermallyinsulated so that heat 56 remains trapped within interior region 82,thereby promoting efficient and uniform heating of the substrate.

V. Preheating Radiation Beam Embodiment

With reference now to FIG. 1C, in another example embodiment, apparatus8 includes a preheating optical relay system 140 having a preheatingradiation source 142 and a relay lens 143 arranged along an optical axisA2. Preheating radiation source 142 is one that emits radiation 147 thatis applied to relay lens 145 with preheating radiation beam 150therefrom used to preheat the substrate just before it is heated by theannealing radiation beam. Radiation 147 has a wavelength that is readily(substantially) absorbed by 100 μm or less of silicon. In an exampleembodiment, preheating radiation source 142 is a laser diode array thatemits preheating radiation 147 having a wavelength of 0.8 μm (800 nm) or0.78 μm (780 nm). An example embodiment of relay lens 143 is describedbelow. Preheating radiation source 142 and relay lens 143 are operablyconnected to controller 32, along with monitors M1 and M2, and stagedriver 29 shown in FIG. 1A, and not shown in FIG. 1C for ease ofillustration.

In operation, preheating radiation source 142 emits radiation 147, whichis received by relay lens 143. Relay lens 143 creates a preheatingradiation beam 150 that forms an image 160 (e.g., a line image) atsubstrate surface 12. Preheating radiation beam 150 is incidentsubstrate surface 12 at an incident angle θ₁₅₀ as measured relative tosubstrate surface normal N.

In one example embodiment, image 30 formed by annealing radiation beam20 and image 160 formed by preheating radiation beam 150 are situatedside-by-side on substrate surface 12, as shown in FIG. 1C. Thus,preheating radiation beam 150 acts to locally preheat the portion orregion of the substrate just in front of the portion being irradiated byannealing radiation beam 20. Arrow 22′ illustrates the movement ofsubstrate 10 (e.g., via movable chuck 28; see FIG. 1), which in anexample embodiment is moved under fixed radiation beams 20 and 150 (orequivalently, fixed images 30 and 160) to effectuate scanning of thesebeams (or images).

In another example embodiment, preheating radiation beam 150 andannealing radiation beam 20 partially overlap, e.g., at the 1/e²intensity contours of the respective beam intensity profiles, asillustrated in FIG. 4A

FIG. 4B is a close-up cross-sectional view of an example embodiment ofthe substrate being irradiated by beams 20 and 150. FIG. 4B illustrateshow heat 166 from preheating radiation beam 150 imaged in front ofannealing radiation beam 20 promotes absorption of the annealingradiation beam near the top surface of the substrate. Heat 166 frompreheating radiation beam 150 diffuses into substrate 10 in front ofannealing radiation beam 20. As the radiation beams move relative to thesubstrate as indicated by arrows 22′, annealing radiation beam 20 passesinto the region (i.e., substrate portion) previously heated bypreheating radiation beam 150. This process is used to raise thetemperature of the substrate at and near the substrate surface to abovethe critical temperature T_(C). This allows annealing radiation beam 20to be efficiently absorbed in the substrate, as indicated by absorbedannealing radiation beam 20′ (dashed line). The relatively rapidabsorption of annealing radiation beam 20′ within substrate 10 nearsubstrate surface 12 serves to quickly raise the temperature of thesubstrate surface to a maximum at the trailing edge of the annealingradiation beam, up to an annealing temperature T_(A) (e.g., about 1600°K.). This leads to annealing of select regions formed in the substrate,e.g., by activating dopants implanted into the top surface of thesubstrate.

VI. Substrate Temperature Plots

FIG. 5 is a plot of the maximum substrate temperature T_(MAX) (° C.)created by irradiating a heavily doped silicon substrate with 10.6 μmradiation, as a function of the incident power P_(I) (W/cm) of theradiation. A two-dimensional, finite-element simulation program was usedto derive this data. The simulation assumed an infinitely long annealingradiation beam. Thus, the beam power is measured in Watts/cm rather thanwatts/cm². The simulation also assumed annealing radiation beam 20 had aGaussian beam profile with a Full-Width Half-Maximum (FWHM) of 120 μm,and was scanned across the substrate upper surface 12 at a speed of 600mm/s, producing a dwell time of 200 μs. Here, “dwell time” is the lengthof time image 30 formed by annealing radiation beam 20 resides over aparticular point on substrate surface 12. In this case, the plot showsan approximately linear relationship between the incident power P_(I)and the maximum substrate temperature T_(MAX). Because thetwo-dimensional model assumed that annealing radiation beam 20 isinfinitely long, there was no additional energy loss at the ends of lineimage 30. A finite beam length would result in some additional heat lossat the ends of the beam and therefore result in lower maximumtemperatures for a given incident power level P_(I).

FIG. 5 shows that in an absorbing (i.e., highly doped) substrate, anincident power P_(I) of about 500 W/cm is required to bring the maximumsubstrate surface temperature T_(MAX) from ambient up to 427° C. for aspecific set of conditions. This can be compared to about 1150 W/cm totake the temperature up to the melting point of silicon at 1410° C. forthe same set of conditions.

The relationship shown in FIG. 5 is a good approximation for apreheating radiation beam 150 having the same width and dwell time asannealing radiation beam 20. Thermal diffusion is the dominant mechanismfor distributing heat in the substrate in both cases. A peak substratetemperature T_(MAX) of 400° C. does not produce nearly the sameabsorption of annealing radiation beam 20 as a uniform substratetemperature T_(S) of 400° C. because the former temperature distributionfalls to ambient within the substrate in a distance roughly equal to thethermal diffusion length L_(D).

FIG. 6 is a plot of the maximum substrate temperature T_(MAX) (° C.) asa function of the initial substrate temperature T_(I) for two differentincident powers P_(I) of annealing radiation beam 20 of wavelength 10.6μm for an undoped silicon substrate. This was also derived from atwo-dimensional finite element model. For temperatures below about 327°C., the incident radiation produces almost no effect, and the maximumtemperature T_(MAX) is almost equal to the initial substrate temperatureT_(I). In other words, annealing radiation beam 20 passes throughsubstrate 10 and does not appreciably heat the substrate. However, at aninitial substrate temperature T_(I) somewhere between 377° C. and 477°C., appreciable absorption of annealing radiation beam 20 occurs,depending on the amount of incident power P_(I) in the annealingradiation beam. The result is a sharp increase in the maximum substratetemperature T_(MAX). Once the high-absorption, high-temperaturetransition has occurred, further irradiation by annealing radiation beam20 increases the maximum temperature T_(MAX) linearly.

Note that the units of power used in the plots of FIGS. 5 and 6 is Wattsper centimeter (W/cm). This power refers to the power per unit length ofthe scanning image 30 (e.g., a line image) contained between the halfpower points. Thus, a power of 1150 W/cm in image 30 having a width of120 μm corresponds to an average intensity of 95,833 W/cm²

The temperature that must be generated by preheating radiation source142 to heat the substrate to the critical temperate T_(C) in order tocreate the self-sustaining annealing condition can be estimated frominformation in the plot of FIG. 6. The plot therein indicates that whena substrate reaches a uniform temperature T_(I) of about 427° C. thereis a sudden increase in the substrate temperature T_(MAX) indicatinginitiation of the self-sustaining annealing condition. If a laser diodesource is used to provide the necessary preheating, then a significantlyhigher temperature is to be expected since the diode source produces anon-uniform temperature distribution that falls to ambient in about onethermal diffusion length.

FIG. 7 is a plot of the absorption length L_(A) (μm) of 780 nm radiationin un-doped silicon as a function of substrate temperature T_(S) (° C.).The absorption characteristics at 800 nm are very similar to that at 780nm. As can be seen from the plot, even at room temperature theabsorption length L_(A) is about 10 μm, which is short enough to ensureeffective heating of the substrate surface region and a temperaturedistribution determined primarily by thermal diffusion for time scalesof 200 μs and above.

In order to obtain efficient absorption of a CO₂ laser beam (asannealing radiation beam 20) in an undoped silicon substrate having anon-uniform temperature distribution, such as that created by a laserdiode source (as used to generate preheating radiation beam 150), atemperature corresponding to an absorption length of about 100 μm isestimated. This is achieved with a peak substrate temperature T_(MAX) ofabout 550° C. Referring again to FIG. 5, a maximum substrate temperatureT_(MAX) of 550° C. would require preheating radiation beam 150 to have apower of about 600 W/cm (50,000 W/cm²).

VII. Determining the Preheating Radiation Beam Power

In practice it is a simple matter to determine the minimum power inpreheating radiation beam 150 needed to achieve efficient coupling ofannealing radiation beam 20 to substrate 10. In an example embodiment,with annealing radiation beam 20 set to a power level sufficient toanneal an absorbing substrate, a substrate substantially nonabsorbent atthe wavelength of annealing radiation beam 20 at room temperature isirradiated with preheating radiation beam 150 and with annealingradiation beam 20. The power level of the preheating radiation beam 150is increased until annealing temperatures are detected in the substrate.This may be accomplished, for example, by measuring the substratetemperature with temperature monitor M2 shown in FIG. 1A.

The transition from little or no coupling of the annealing radiationbeam with the substrate to efficient coupling at the substrate surfaceis typically quite sudden. If the substrate temperature T_(S) is too lowthere will either not be a transition to annealing temperatures or asudden transition to the substrate melting point will occur. As thesubstrate temperature is raised further there will be a narrow range ofannealing power levels that permit stable operation below the meltingtemperature. A further increase in substrate temperature increases therange of annealing power levels and the corresponding range of annealingtemperatures. Thus, there is no sharply defined power level ofpreheating radiation beam 150 to initiate the absorption transition ofannealing radiation beam 20 in the substrate, or alternatively, thatleads to the annealing temperatures in the substrate. However there is aminimum practical power level below which the desired range of annealingtemperatures cannot be reliably achieved. In an example embodiment,preheating radiation beam 150 is set to a power level slightly abovethis minimum power level needed to ensure that the annealing radiationbeam is efficiently absorbed by the substrate and that a large range ofannealing temperatures are readily accessed.

In an example embodiment, the amount of power P_(I) in preheatingradiation beam 150 required to initiate the self-sustaining annealingcondition is that necessary to produce a maximum substrate temperatureT_(MAX) of 550° C. Assuming a 200 μs dwell time, the graph on FIG. 5indicates that this corresponds to an incident power of about 600 W/cm.However, obtaining an intensity of say 600 W/cm in a preheatingradiation beam 150 that produces an image 160 having a width comparableto that of annealing radiation beam image 30 is not as easy as it mightfirst appear. In an example embodiment, it is desirable that preheatingradiation beam 150 have an angle of incidence θ₁₅₀ at or near theBrewster's angle for silicon, which is about 75°. This angle minimizesthe reflected radiation and equalizes the energy absorbed in thesubstrate for the types of structures likely to be present on thesubstrate. At an incident angle θ₁₅₀ of about 75°, preheating radiationbeam 150 is smeared out at substrate surface 12 and covers an areaincreased by a factor of about 4, and the intensity is reducedproportionally.

The total power in preheating radiation beam 150 can be increased, forexample, by making the preheating source larger, e.g., by addingadditional rows of laser diodes. However, this increases the width ofpreheating radiation beam 150 proportionally. An increased preheatingradiation beam width increases the dwell time and the thermal diffusiondepth, which further increases the power required to attain a givenmaximum temperature. Thus, relay lens 143 needs to be designed so thatit can provide a preheating radiation beam 150 having sufficientintensity to heat the substrate to within the critical temperate rangeusing available preheating radiation sources 142. An example of such arelay according to the present invention is described immediately below.

VIII. Example Embodiment of Optical Relay System

FIGS. 8A and 8B are respective cross-sectional views of an exampleembodiment of optical relay system 140 and substrate 10. FIG. 8A is aview in the Y-Z plane, and FIG. 8B is a view in the X-Z plane. In bothFIGS. 8A and 8B, the relay has been divided into two parts in order tofit on the page and the lens element with surfaces S13 and S14 is shownin both parts.

In the example embodiment, preheating radiation source 142 includes a2-dimensional laser diode array, such as the LightStack™ 7×1/L PV arrayavailable from Coherent Semiconductor Group. 5100 Patrick Henry Drive,Santa Clara, Calif. 95054. The LightStack™ array contains 7 rows ofwater-cooled laser diodes each 10 mm long and stacked on 1.9 mm spacing.Each row of diodes is capable of emitting 80 watts of optical power.Relay lens 143 includes an object plane OP (where preheating radiationsource 142 is arranged), an image plane IP (where substrate 10 isarranged), and optical axis A2 connecting the image and object planes.

In an example embodiment and as discussed above, relay lens 143 isdesigned to create a preheating radiation beam 150 that forms an image160 (e.g., a line image) that is scanned over substrate 10. The scanningof image 160 can be accomplished in any number of ways, such as bymoving chuck 28 (via movable stage MS) relative to relay lens 143 (FIG.1C). Locally irradiating substrate 10 with image 160 is preferred toirradiating the entire substrate at once because it is much easier toachieve the high beam intensity needed to heat the substrate over arelatively small image area. Thus, the local preheating provided byrelay lens 143 must be synchronized with irradiating the substrate withannealing radiation beam 20.

Since the emission characteristics of laser diodes are anisotropic andthe spacing between adjacent diodes is greatly different in the X and Yplanes, relay lens 143 needs to be anamorphic in order to efficientlyform image 160 at substrate 10. Furthermore, to achieve the requiredintensity in image 160 at substrate 10, a relatively high numericalaperture at image plane IP is necessary.

Thus, with reference also to FIGS. 9A and 9B, relay lens 143 includes inorder from preheating radiation source 142 and along optical axis A2, acylindrical lens array 200 having lenslets 201 corresponding to thenumber of rows of laser diodes 198 making up preheating radiation source142. Cylindrical lens array 200 has power in the X-Z plane and acts tocollimate each preheating radiation beam 147 emitted from radiationsource 142 in the X-Z (FIG. 9A) plane, while allowing the radiation tohave a 10° cone angle in the Y-Z plane (FIG. 9B). The combination of thediode array and the cylindrical lens array serves as the input to ananamorphic relay, which reimages the cylindrical lens array onto thesubstrate.

Table 1 sets forth the lens design data for an example embodiment ofrelay lens 143 as illustrated in FIGS. 8A and 8B.

With reference again to FIGS. 8A and 8B, relay lens 143 consists of twoimaging sub-relays R-1 and R-2 in series with a common intermediateimage plane IM. Sub-relay R1 is an anamorphic relay employing mainlycylindrical lens elements with substantially different powers in the Y-Zand X-Z planes, while subrelay R-2 is a conventional relay employingspherical elements and having a demagnification ratio of 1:6. Theanamorphic relay R-1 has a 1:1 magnification ratio in the Y-Z plane, anda 1:10 demagnification ratio in the X-Z plane. The relay lens 143 istelecentric at the object plane OP and image focal plane OP.

Telecentricity at both the object plane OP and the image plane IP isachieved with a spherical field lens 202 (surfaces s1–s2) and acylindrical lens 204 (surfaces s3–s4) arranged immediately adjacentpreheating radiation source 142. The cylindrical lens 204 has power onlyin the Y-Z plane and forms a pupil image in the Y-Z plane at s5. Nextare two cylindrical lenses, 206 and 208 (surfaces s6–s9) with power inthe Y-Z plane that reimage the diode array at 1:1 at the intermediateimage plane. Surface s10 identifies a pupil plane in the X-Z plane.These are followed by a pair of cylindrical lenses, 210 and 212(surfaces s11–s14) having power in the X-Z plane that also reimage thediode array at the intermediate image plane at a demagnification ratioof 10:1. The intermediate image is reimaged on the final image plane bya group of spherical lenses, 214–222 (surfaces s15–s24) that form asub-relay with a demagnification ratio of 6:1. Thus the relay has anoverall demagnification of 6:1 in the plane containing the rows ofdiodes, and 60:1 in the plane normal to each row of diodes.

The 6:1 demagnification ratio in the Y-Z plane reduces the 10 mm size ofthe uncollimated (slow-axis) of preheating radiation source 142 from 10mm at object plane OP to 1.67 mm at image plane IP. Also, in the sameplane the 10° cone angle of radiation emitted from the preheatingradiation source 142 at object plane OP is increased to 60° at imageplane IP.

The demagnification in the X-Z plane is 60:1. Thus, the 11.4 mmdimension (as measured in the X-direction across 7 rows of diodes) ofthe laser diode array making up effective source 220 at object plane OPis reduced to 0.19 mm at image plane IP. In addition, the 1° FWHMangular spread in the collimated beam at effective source 220 isincreased to a 60° cone angle at image plane IP.

If it is assumed that the overall efficiency of transmitting preheatingradiation 147 from radiation source 142 at object plane OP to substrate10 at image plane IP is 50% (including reflection losses at substratesurface 12), then relay lens 143 of FIGS. 8A and 8B is capable ofbringing 280 W into image 160. For the example image 160 dimensions of1.6 mm by 0.19 mm, this achieves a power density of 921 W/mm². At normalincidence (θ₁₅₀=0°), this power density will raise the temperature of aroom-temperature (i.e., ˜20° C.) silicon substrate 10 by about 500° C.to a temperature near 520° C. assuming a dwell time of about 0.2 ms.This is above the critical, uniform temperature T_(C) of 400° C.required to initiate the self-sustaining annealing condition and is inthe right range for a non-uniform temperature distribution such as thatproduced by a diode array image 160 located just in front of theannealing laser image 30. In this case, it is assumed that preheatingradiation beam 150 precedes (i.e., is scanned in front of) annealingradiation beam 20. In this way, the maximum temperature T_(MAX) createdby the preheating radiation beam is achieved just prior to annealingradiation beam 20 irradiating the same preheated portion of thesubstrate. In an example embodiment, the relative position of thepreheating and annealing radiation beams are reversed each time the scandirection is reversed so that the preheating radiation beam alwaysprecedes the annealing radiation beam.

IX. Radiation Beam Scanning and Orientation

As mentioned above, in an example embodiment, image 160 formed bypreheating radiation beam 150 is scanned over substrate 10. Inconjunction therewith, image 30 formed by annealing radiation beam 20 isalso scanned over the substrate so that it is incident on the substrateportion(s) preheated by the preheated radiation beam.

In example embodiments, scanning is carried out by moving the substratein a spiral, raster or boustrophedonic pattern. In a boustrophedonicscanning pattern, the scan direction is reversed and the cross-scanposition incremented after every scan. In this case, as mentioned above,it is necessary to change the relative positions of preheating radiationbeam 150 and annealing radiation beam 20 between each scan. In anexample embodiment, this is accomplished by shifting the position of theentire relay lens 143. Where annealing radiation beam 20 is about 120 μmwide (FWHM) and preheating radiation beam 250 is about 190 μm wide(top-hat profile), then relay lens 143 needs to be moved by about twicethe distance between the beam centers or about 393 μm in a directionparallel to the scan direction. This is accomplished, for example, via asignal from controller 32, which is operatively connected to preheatingrelay lens 143 to effectuate movement of the relay lens (FIG. 1C). In alike manner, controller 32 controls the focus of preheating radiationbeam 150 by adjusting the focus, tip, and tilt parameters of thesubstrate prior to scanning.

As discussed in aforementioned U.S. patent application Ser. No.10/287,864, it is desirable to have annealing radiation beam 20 beincident substrate 10 at an incident angle at or near the Brewster'sangle, and be P-polarized. This is because the film stacks likely to beencountered on a substrate during annealing have a low reflectivity anda small variation in reflectivity under these conditions.

In an example embodiment, preheating radiation beam 150 is arranged sothat it strikes the substrate at incident angle θ₁₅₀, at or near theBrewster's angle in a manner similar to that of annealing radiation beam20. Generally this angle reduces the variation in reflectivity betweenthe different film stacks likely to be found on a substrate prior to theactivation (annealing) step. However, while this beam orientation(angle) works very well at the annealing wavelength, it is not aseffective at the wavelength used for preheating. The rough equivalencebetween the preheating radiation beam wavelengths and the thickness ofthe films used to make semiconductor structures (e.g., devices 14, suchas transistors) leads to a greater variation in substrate reflectivityat all angles of incidence. Furthermore, an incident angle θ₁₅₀ at ornear Brewster's angle spreads image 160 over an area 3 or 4 times biggerthan at normal incidence (i.e., θ₁₅₀=0°) and lowers the power density acorresponding amount. If the scan rate is left unchanged, since it isusually set by the annealing radiation beam geometry, then the maximumtemperature is also reduced.

One of the problems with operating at normal incidence or near normalincidence is that the reflected proportion of the radiation can be quitehigh and can cause serious damage if it returns to the radiation source(e.g., diode array). FIGS. 10A and 10B are schematic diagramsillustrating example embodiments of preheating relay optical system 140for decreasing the amounts of preheating radiation reflected orscattered back to the preheating radiation source 142 (FIG. 1C). Withreference to FIG. 10A, in a preferred embodiment, preheating radiationbeam 150 has a normal incident angle of θ₁₅₀=0°. A normal angle ofincidence results in an amount of preheating radiation beam 150 beingreflected from the substrate (the reflected preheating radiation isdenoted 150R) and being transmitted back toward the preheating radiationsource 142. If reflected preheating radiation 150R makes it back topreheating radiation source 142, it may accelerate the source's time tofailure. Where emitted preheating radiation 147 is polarized (such as isthe case with laser diodes), then in an example embodiment, the amountof reflected preheating radiation 150R returning to the preheatingradiation source is reduced by arranging a polarizer 143P aligned withthe polarization direction of the preheating radiation beam, and aquarter-wave plate 143WP located between the polarizer and thesubstrate. The quarter wave plate converts the radiation traveling fromthe polarizer to the substrate into circularly polarized radiation atthe substrate. Any radiation returning from the substrate is convertedback to linearly polarized radiation after passing through the quarterwave plate. However, the direction of polarization of the returningradiation is orthogonal to the original direction. Thus the returningbeam is not transmitted by the polarizer and does not reach the laserdiode array.

With reference now to FIG. 10B, even if the incidence angle θ₁₅₀ ischosen off-normal incidence so that reflected (specular) preheatingradiation 150 cannot return to the preheating radiation source,scattered (or non-specular) preheating radiation 150S returning topreheating radiation source can present a problem. Even a small amountof radiation returned to some types of preheating radiation sources(such as lasers) can cause operational instability. Also, it isdesirable to employ p-polarized preheating radiation when operating offof normal incidence in order to increase the proportion of radiationthat is absorbed in the substrate and to reduce the variation inabsorption caused by the various structures on the substrate.

Thus, in an example embodiment, the amount of preheating radiation 150Sthat returns to the preheating radiation source 142 is reduced by addinga polarizer 143P and a Faraday rotator 143F downstream of relay lens143. The Faraday rotator 143F is located between the polarizer 143P andsubstrate 10. In operation, the Faraday rotator rotates the polarizationof the preheating radiation beam 150 by 90° after two passes through therotator, and the polarizer blocks the polarization-rotated preheatingradiation 150S from returning to preheating radiation source 142.Operating optical relay system 140 such that preheating radiation beam150 is off of normal incidence also facilitates measuring the power inreflected preheating radiation beam 150R, which is a useful diagnostic.

Measurements of the power in incident preheating radiation beam 150 andreflected preheating radiation 150R can be used to calculate the powerabsorbed by the substrate 10. This is then used to estimate the maximumtemperature produced by preheating radiation beam 150. By keeping theabsorbed power in preheating radiation beam 150 above a certain minimumthreshold, preheating sufficient to trigger strong absorption of theannealing radiation beam 20 by the substrate is assured.

In the foregoing Detailed Description, various features are groupedtogether in various example embodiments for ease of understanding. Themany features and advantages of the present invention are apparent fromthe detailed specification, and, thus, it is intended by the appendedclaims to cover all such features and advantages of the describedapparatus that follow the true spirit and scope of the invention.Furthermore, since numerous modifications and changes will readily occurto those of skill in the art, it is not desired to limit the inventionto the exact construction and operation described herein. Accordingly,other embodiments are within the scope of the appended claims.

1. An apparatus for performing laser thermal annealing of a substratehaving a surface, comprising: a laser capable of generating continuousannealing radiation having a wavelength that is not substantiallyabsorbed by the substrate at room temperature; an annealing opticalsystem adapted to receive the annealing radiation and form an annealingradiation beam that forms a first image at the substrate surface, andwherein the first image is scanned across the substrate surface; and aheating device for heating at least a portion of the substrate to acritical temperature such that the annealing radiation beam incidentupon said portion is substantially absorbed near the surface of thesubstrate at said portion during scanning; wherein the heating deviceincludes: a preheating radiation source adapted to emit preheatingradiation of a wavelength that is substantially absorbed by thesubstrate at room temperature; and a relay system adapted to receive thepreheating radiation from the preheating radiation source and form apreheating radiation beam that forms and scans a second image over thesubstrate surface to preheat a portion of the substrate that is in frontof or that partially overlaps the scanned first image.
 2. The apparatusof claim 1, wherein the annealing radiation beam wavelength is greaterthan 1 μm.
 3. The apparatus of claim 1, wherein the substrate issupported by a movable stage adapted to achieve said scanning by movingthe substrate relative to the annealing radiation beam.
 4. The apparatusof claim 1, wherein the annealing radiation has a wavelength of 10.6 μm.5. The apparatus of claim 1 wherein the substrate is undoped or lightlydoped silicon.
 6. The apparatus of claim 1, wherein the first imageformed by the annealing radiation is a line image.
 7. The apparatus ofclaim 1, wherein the heating device includes a heated chuck adapted tosupport and heat the substrate to the critical temperature.
 8. Theapparatus of claim 7, wherein the heating device further includes a heatshield that reflects heat emitted from the substrate back to thesubstrate.
 9. The apparatus of claim 1, where the heating deviceincludes a heated enclosure adapted to surround the substrate and toheat the substrate to the critical temperature.
 10. The apparatus ofclaim 1, wherein the relay system includes: a relay lens.
 11. Theapparatus of claim 10, wherein the second image is a line image.
 12. Theapparatus of claim 10, wherein the preheating radiation beam has awavelength of 780 nm or 800 nm.
 13. The apparatus of claim 10: whereinthe relay lens is arranged such that the preheating radiation beam isincident on the substrate at normal incidence; and further includes: apolarizer; and a quarter-wave plate; wherein the polarizer andquarter-wave plate are arranged in the preheating radiation beam toreduce an amount of preheating radiation reflected from the substratefrom reaching the preheating radiation source.
 14. The apparatus ofclaim 10, wherein a polarizer and a Faraday rotator are arranged in thepreheating radiation beam to substantially prevent preheating radiationfrom returning to the preheating radiation source.
 15. The apparatus ofclaim 10, wherein: the preheating radiation source is an array of laserdiodes; and the relay lens is anamorphic and forms the second image as aline image of the array of laser diodes at the substrate.
 16. Theapparatus of claim 10, wherein the relay lens includes elements that areadjustable to maintain the second image in focus.
 17. The apparatus ofclaim 1, further including: a chuck that supports the substrate; amovable stage that supports the chuck; and a stage driver operativelyconnected to the movable stage to selectively move the stage toselectively move the substrate to effectuate said scanning.